Nuclear chain fission reactions and the reactors in which such reactions are accomplished are now well known. In general, a nuclear reactor is made up of a chain reacting assembly including nuclear fuel material contained in fuel elements having various geometric shapes such as plates, tubes, or rods. These fuel elements are usually provided with a corrosion-resistant non-reactive heat conductive layer or clad on their external surfaces. In power reactors, these elements are usually grouped together at fixed distances from one another in a coolant flow channel or region forming what is termed a fuel bundle. A sufficiently large number of such bundles are combined together in the chain reacting assembly or core to permit a controllable self-sustained nuclear fission chain reaction. The reactor core is enclosed within a container through which the reactor coolant is circulated. In thermal neutron spectrum reactors, a neutron moderator is also provided, and in some cases this moderator may also perform as the reactor coolant. The known boiling water and pressurized water reactors are examples of such thermal reactors.
The nuclear fuel material may contain fissionable atoms such as U-233, U-235, Pu- 239, or Pu-241, and this material may be in elemental or compound form. Upon absorption of a neutron by the nucleus of such a fissionable atom, a nuclear disintegration frequently results. This produces on the average two fission product atoms of lower atomic weight and of great kinetic energy. Also released in such a disintegration are several neutrons of high energy. For example, in the fission of U-235 atoms, light fission product atoms of mass number ranging between 80 and 110 and heavy fission product atoms of mass number ranging between 125 and 155 are produced. On the average, 2.5 neutrons per fission event are released. The total energy released approaches 200 mev. (million electron volts) per fission.
The kinetic energy of the fission product atoms as well as that of the fission neutrons is quickly dissipated producing heat in the fuel elements of the reactor. Some additional heat is generated directly in the reactor structural materials, in the coolant, and any moderator present, due to radiation effects. If there is one net neutron remaining on the average from each fission event and this neutron induces a subsequent fission event, the fission reaction becomes self-sustaining and is thus called a chain reaction. Heat generation may be maintained and the heat is removed by passing a coolant fluid through heat exchange relationship with the fuel elements. The fissionable atoms are thus gradually consumed. Some of the fission product atoms produced are strong neutron absorbers (fission product poisons). Thus the fission reaction tends to decrease and cannot be maintained indefinitely at a given level.
In some nuclear reactor fuel elements, fertile atoms such as U-238 may be included in addition to the above noted fissionable atoms. A fairly common currently used nuclear power reactor fuel material consists of enriched uranium dioxide (UO.sub.2) in which approximately 2.0% of the uranium atoms are U-235 which are readily fissionable by thermal neutrons, while the approximate remainder of 98% is U-238 which is not so fissionable to any significant degree. In the course of operating a reactor fueled with such fissionable and fertile atoms, the fissionable atoms (U-235) originally present are gradually consumed and simultaneously neutron irradiation of the fertile atoms (U-238) converts a part of them into additional fissionable atoms (Pu-239). Initially, the concentration of these newly created fissionable atoms gradually rises with irradiation and then approaches an equilibrium value. These atoms are also readily fissionable by thermal neutrons and thus contribute to the maintenance of the chain fission reaction so that the reaction may be continued longer than would have been the case if only the original charge of fissionable atoms were available.
Since the rate at which fissionable atoms are created by fertile atom conversion is (except in the breeder-converter type of reactor of special design) always less than the rate at which the original fissionable atom charge is consumed during operation, the reactor can maintain this heat generation at a given power level for only a finite length of time. Thereafter, the maximum power level at which the reactor can be operated must be decreased and finally the reactor must shut down for refueling. Some or all of the irradiated fuel bundles are removed and replaced with new fuel bundles having a higher concentration of fissionable atoms and no fission product poisons. The reactivity of the refueled reactor core is higher and the original power level can thus be restored.
The irradiated reactor fuel removed from the reactor ordinarily contains a valuable unconsumed quantity of the original fissionable material as well as a significant quantity of fissionable material (the fissionable atoms) converted from any fertile material (the fertile atoms) which may have been a component of the original fuel. Irradiated fuel also may contain fission products (the fission product atoms) or transuranic isotopes (or both) which are of substantial value. In addition to plutonium referred to above, one such transuranic is the neptunium isotope Np-237, which is formed from neutron irradiation of U-235 and U-238 in accordance with the following reactions: ##STR1## While Np-237 may have other uses, one current use is in the production of Pu-238 by further neutron irradiation in accordance with the following reactions: ##STR2## Pu-238 is a long lived (89 year half-life) energetic alpha particle emitter, the radioactive decay of which yields thermal energies at rates sufficient to power direct thermal to electrical energy conversion devices.
Accordingly it is highly desirable to reprocess the irradiated fuel to recover and separate the fissionable and fertile materials for reuse, and to recover transuranic isotopes such as plutonium for use in reactor fuels and Np.sup.237 for use in production of Pu.sup.238, or for other uses.
One particularly advantageous irradiated fuel recovery process is described and claimed in U.S. Pat. No. 3,374,068 issued Mar. 19, 1968 in the names of O. D. Erlandson and B. F. Judson. This patent presents an improved chemical process capable of separating transuranic irradiation products from one another and from uranium and fission products. In this process the irradiated fuel is dissolved in acid to form an acid solution. The uranium, plutonium and neptunium are separated without partition from the fission products in the irradiated nuclear fuel solution by extraction with an organic solvent thus forming a product stream and a fission products stream. The fission products stream is dehydrated and calcined and these materials are recovered as an oxide mixture. The uranium and transuranium products in the product stream are recovered from the product stream as a mixture with substantially reduced amounts of fission products. The transuranics are separated from the uranium, and from each other if desired, by contact with and elution from anion exchange resins. The uranium stream is dehydrated and calcined to produce uranium oxide containing trace fission oxides and the substantially plutonium free oxide mixture is fluorinated so that uranium hexafluoride is recovered as a product substantially free of fission product fluoride.
The present invention is directed to an improvement of the process of U.S. Pat. No. 3,374,068 in which the decontamination factor of niobium is enhanced to provide improved purity of the recovered actinide products.